(1) Field of the Invention
The present invention relates to a developing device capable of being supplied with toner and an image forming apparatus and a toner supply control method using the same device, in particular relating to a developing device using a developer containing a toner and a magnetic carrier, which can make exact control of the amount of toner supply and an image forming apparatus and a toner supply control method using the same device.
(2) Description of the Prior Art
The image forming apparatus using the electrophotography is, in general, an apparatus that forms images on recording media by performing the steps of charging, exposure, development, transfer, fixing, cleaning and charge erasing.
In the charging step, the surface of a photoreceptor that is rotationally driven is uniformly electrified by a charging device. In the exposure step, the electrified photoreceptor surface is illuminated with a laser beam from an exposure device to form an electrostatic latent image corresponding to image data. In the developing step, the electrostatic latent image on the photoreceptor surface is developed by a developing device to form a toner image. In the transfer step, the toner image formed on the photoreceptor surface is transferred to a recording medium by a transfer device. In the fixing step, the transferred toner image is heated and pressed by a fusing device so as to be fixed to the recording medium.
In the cleaning step, the residual toner remaining on the photoreceptor surface after toner transfer is removed by a cleaning device so that the photoreceptor surface is cleaned. The toner removed at this time is collected into a predetermined collecting portion. In the charge erasing step, the photoreceptor surface after cleaning is cleared of residual charge by a charge erasing device to prepare for a next image forming operation.
In the thus constructed image forming apparatus, a mono-component developer consisting of a toner only or a dual-component developer consisting of a toner and a carrier is used as the developer to develop the electrostatic latent image.
Since the mono-component developer does not need to have an agitating mechanism for mixing toner and carrier to be uniform, use of the mono-component developer is advantageous in making the configuration of the developing device simple and compact, though there is a drawback that the static charge on the toner is unlikely to be stabilized.
On the other hand, since the dual-component developer needs to have an agitating mechanism for mixing the toner and carrier to be uniform, there is a drawback that the developing device becomes complicated and enlarged. However, since the dual-component developer has the advantage that it presents stable charge performance and suitability to high-speed machines, it is often used for high-speed image forming apparatuses and color image forming apparatuses.
Recently, in order to meet the demands of the users for energy saving and high-quality image printout, micro toners having a low softening temperature and a volume mean diameter as low as 5 μm to 9 μm have become often used. Though the micro-sized toner of this kind is designed to be fixable at a low temperature and is effective in enhancing resolution and reducing granulation to achieve improved image quality.
The toner in the dual-component developer is consumed every time development is performed, so that a toner supply device for supplying toner into the developing device in accordance with the amount of toner consumption is provided inside the developing device. As the toner supply device for this purpose, there has been a known toner supply device which, for example, includes a screw auger as a toner discharging member that is rotated to convey the toner to a toner discharge port and supply it to the developing device (patent document 1: Japanese Patent Application Laid-open 2001-83802).
However, in the aforementioned toner supply device using the toner discharging member formed of a screw auger, there would occur the phenomenon that lumps of toner residing in the gap of the screw auger suddenly flow out from the toner supply device to the developing device due to vibration etc. of the image forming apparatus even when no toner supply is performed or when the developer is sufficiently high in toner concentration. Accordingly, there has been the problem that the amount of toner supply from the toner supply device to the developing device cannot be controlled exactly.